server: Store max chunk size in database.

server: Store max chunk size in database.

This number is needed up-front for the initialization of the fec data
structures.  Currently we recompute it from the chunk table each time
the file is opened for streaming.

We can only get rid of the chunk table concept if we tell the VSS by
other means how to obtain this information.

Fortunately there is an unused 4-byte field in the on-disk afhi
structure, which is always zero at the moment. This patch starts to
use this field to store the maximal chunk size.

For backwards compatibility, when the afhi structure is loaded from
disk at stream time, we check if the field is zero and recompute the
max chunk size as before in this case.

Although the maximal chunk size is generally only needed on the
server side, for consistence we expose it though a new status item
along with chunk_tv and friends.